Sand or dust..............never use a cloth. Blow that stuff off. Can of compressed air or a blower brush works well.

I always got rid of fingerprints as I suggested...........although they were not French Fry grease coated fingers. Anything real stubborn, a lens cleaning solution and Kodak lens cleaning disposable tissues..........sure. I think I may have used that stuff a couple/three times in 20+ years and usually after a salt spray drenching at the beach.

The least amount of anything contacting the front or rear elements of the lens the better..............I also always used a UV or Skylight filter over the front element.

i've had good luck so far using the Lens Pen system. One 1/2 of it is a soft brush for sweeping off dust, the other side has a solution that takes care of smudges and things like that. Be careful though: people have reported that the cleaning solution side has left a weird residue on their lens, so it's best to buy this from a store that will actually let you test the product or give a demonstration of it.